In the middle of my Ls swap right now. Swapping in an LQ9 with LS3 heads and a pretty mean cam(231/244), will hopefully be making around 540 or so at the flywheel. Trying to decide between the 3.5 and 4 inch mufflex or HAWKS sinister system. I think the 4" would be overkill for right now but I do plan on upgrading power down the road. I'm also going to do a cutout after the Y most likely. I just want to make sure that if I'm going with the 4" I won't be losing too much low end torque since this car will be mainly an autocross car. Also wondering what my options are for a Y, I have Hooker 3" LT headers with 1 7/8 primaries. I'm guessing something custom? Also I'm running a UMI panhard relocation and lowering kit so I don't know how much clearance that'll give me.

 

Free flowing exhaust lifts the entire torque curve. There's no loss of low end torque. Headers can change the shape of the torque curve though.

Flowmaster makes a really good Y-pipe.You can see the various inlet and outlet choices on their website.